Well the results are in.... (the summer results, anyway): For the last week, the Kuuma has been supplying all the hot water for our home, with no problems. Since you last heard from me, I've added a Taco 006e3 circ pump to the heat exchanger loop, rather than moving the water storage tank to the closet on the floor above the heater. I think this has to work better, keeps the plumbing, tanks, and potential leaks in the room with the concrete floor and drain, and has the side effect of preserving my marriage through closet space. An interesting thing - with the pump set at it's lowest setting, which supplies plenty of water flow, without negatively affecting the hot water stratified at the top of the tank, that pump draws a total of 4 watts! I wouldn't thing you could do anything with water with 4 watts of energy, but it does and makes the whole thing work! Not the cheapest pump you could buy, but an impressive piece. It feels like quality too!
Besides adding some temporary, crude insulation to the whole circ loop, another thing I've done in the time is to seal up all the leaks of really hot air. Many were in my plenum connection to the stove, and the heat exchanger insertion point, but one that I think every owner should look at is the front removable cover area. From the factory the stove had an 1/8 gap there that shot out a lot of 200 degree air that is better used elsewhere unless you really need to heat the room the furnace is in. Plugging the leaks made a HUGE difference in the heat in the furnace room, and seemed to make a real difference in the amount heat delivered to the water. Being the middle of July, heating the house is not really a thing, much, so I can't really tell from observation, about the delivered air, but it has to make a substantial difference there too.
OK, I know many of you are wanting more detail on the results, so here, at the risk of boring some, is more of that: If somebody wants even more detail, please ask. I've been firing the stove usually for about 4 hours an evening, every other day. That has worked to supply all our hot water needs, using a diet of dry but ugly, rotten, short, or misshapen wood debris that gets produced during the summer cutting season. For those who are going to guess that we're living some kind of minimalist lifestyle - no - it's just my wife and myself, and we have a reasonably low flow shower head, but a 20 year old dishwasher that runs frequently, since restaurants and take-out are not options in the middle of nowhere, plus a super capacity top load washer, that almost always washes on warm, so though we give some thought to conservation of hot water, we use our share. I've been learning to adjust the wood I put in the stove according to the starting temperature of the water tank. I'm getting this figured out pretty well now. With the circ pump, the heat exchanger is so efficient at transferring heat, that most of the heat from the stove ends up in the water tank. If there's 60 degree water in the bottom of the tank, the air coming from the duct is not more than 70 degrees, even though the air measured below the heat exchanger might be 150. Of course, at the end of the heating, if the tank is 125, the output air is also about that, not much hotter, as some of the cooler air is bypassed around the heat exchanger.
Today for example, was a large use day. I last fired the stove yesterday morning, topping the tank out at 125 after another heavy use time. I pounded the hot water supply with a load in the dishwasher, showers, and two super loads of laundry since, and to top it off, I also used a lot of somewhat warm water to wash my super sized smelly German Shepard. The last load of laundry left the sensor at the 2/3 tank height reading 64 degrees. The tap water was still hot, but I was too lazy to get an exact temp reading. I fired the stove 2 hours and 15 minutes ago, putting my guess at the right amount of wood in, and leaving the temp control on HI. When I checked a few minutes ago, the water temp was up to 100. The nice thing, is it is really a set an forget process, once you learn how much wood you need. Because I have yet to decide how best to control the circ pump, I still have to unplug it at the end of the heating cycle, but that's temporary. A possibly interesting side note - while I've tested this process with consistently 60 degree return air, and closed ducting, in preparation for winter; for the summer, I've found that opening the discharge and return air ducts in the furnace room makes the process faster an more efficient for just heating water. Because the heat transfer to the water is so efficient, tonight the temp of the furnace room had risen from 62 when I started the stove, to just 88 a few minutes ago.
I am using a fan control, rather than the factory snap switch to control my fan. And with the ducting open, allowing for maximum convection, the fan cycles somewhat even with the stove on high. To primarily heat water, I'm using a bit higher settings on the fan control than would be desirable to get the most hot air out of the stove. But, I'm not using temperatures any higher than I observed in firing the stove in the bone stock configuration, as the standard snap switch's inaccuracies allowed for some pretty large variations. I've smoothed out the peaks and valleys, and that, combined with taking the air from the hot parts of the stove, has given me the temps I need to heat water. The only thing that I've been able to see that's changed noticeably is the exhaust right at the stove collar rises about 15-20 degrees when I'm running the stove in the extra hot furnace room. I attribute that to the warmer return air not being quite as good at the final cooling of the gasses, plus the somewhat higher fan setting. It still tops out at 220 though - cool enough to touch very briefly.
I just checked again, and the water temp has risen from 100 to 108 in the last 45 minutes I've been sitting here, and the furnace room is still sitting at 88 with the door cracked open. I did decide (probably wrongly!) that I had slightly underestimated the wood, and tossed another small scrap in on the coals for good measure.
So, in the end, I'm really happy with the results! If I can get enough hot water to run my home in the middle of summer (today is July 21) firing every other day, about, I'm sure this will work in the winter. Oh, I guess I forgot to mention, I've been maintaining the house temperature between 60 and 65 degrees the whole time, so no, I'm not overheating the house to do this (no, I don't have A/C). And, yes, it is the freakishly cold climate I live in that allows me to do this through the summer, but I would think it would also work well for someone in a more normal climate through the shoulder seasons. One of my concerns was to be able to get temps high enough for safety (legionella, etc), and I've proven I can get that. It's wait and see what I get in the winter, once I'm running the fan to get every ounce of hot air, but I'm reassured that I could at least switch to a higher fan setting say during the overnight hours to get the water temp up regularly to ensure safety. And, clearly, when it comes to raw output of warm enough (100-115) water for a marathon laundry session, this system is far superior in recovery rate to the old one. This makes lots of 100 degree water fast, it just peaks out a lot cooler.
A benefit I've seen, besides the set and forget burn of the Kuuma, is with the circ pump, the water tank is an honest temp, top to bottom. Before, during the summer, I might fire the old stove until I had about 150 at the temp sensor. But using just thermosiphon, the bottom of the tank was somewhat cooler. So, I seem to be getting more mileage out of the 125ish full tank, than I was getting out of the 150 degree old system, which I typically fired daily in the summer. And I think the old stove produced more sometimes unneeded and unwanted hot air to the house during these summer firings.
A big benefit I will see in the winter, again, besides the set and forget nature of the Kuuma, is that the lower temps of the plenum scavenged system can never try to overheat my water tank. I used to have to choose when it was really cold, to either fire another stove, or think of a way to use hot water, as my storage tank climbed above 180 degrees (not that a hot bath on a sub-zero day is a terrible thing to be forced into - just at some point, it's not so productive).
So, as I end this novel, I just checked again. 3 hours and 45 minutes after starting the fire, the fire is waning, with a couple of big red glowing chunks in there, and the air control now on 3. The water tank is at 115 degrees, and the furnace room is 90. I shut the Kuuma computer off. The rest of the house is at 62 and cooling with the doors and windows open and a breeze blowing the 58 degree outside air in. I'll vent the heat from the furnace room in an hour or so when I unplug the circ pump. I'd just let it warm the house, typically, but we're expecting a small heat wave - supposed to get to around 80 the next couple of days and I don't have A/C, so I'm storing some cold
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It's been a bit of an uncertain road, and I still have some tidying up to do (insulation is a very good thing - and I need to do more of that), but I have to say that I'm happy and reassured that the work has been worth it. I'm looking forward to a somewhat warmer and easier winter this year (inside the house, anyway). For now, life is good!